1. Field of the Invention The present invention relates to a radar apparatus installed on a vehicle, and more particularly to such a radar apparatus, which detects a reflector such as a preceding vehicle and manages information indicative of the reflector.
2. Description of the Related Art
For the application in a tracking operation relative to a preceding vehicle and an alarm system for prevention of collision between vehicles, a radar apparatus installed on a vehicle has been developed. In the radar apparatus, a beam such as millimeter wave and laser wave is radiated to a reflector in such a way that a reflection wave is received back from the reflector to detect information on the reflector. For use in this radar apparatus installed on a vehicle, various radars have been developed such as an FM radar which transmits and receives a frequency modulation wave, an AM radar which transmits and receives an amplitude modulation wave, and a pulse radar.
In the radar apparatus installed on a vehicle, namely, in a vehicle radar apparatus for monitoring the front side of the vehicle, a beam having a sharp directivity is mechanically scanned to cover a predetermined angle range in the front of the vehicle. Otherwise, plural beams of the sharp directivity are transmitted in order from a plurality of antennas arranged to direct the beams at slightly different directions from each other. Thus, by performing electronic scanning of the beams, an azimuth data for the reflector and a distance to the reflector may be are obtained. In Japanese Patent No. 2567332, which is granted to the assignee of the present invention, a time divisional radar system is disclosed, in which a beam is electronically scanned. Then, a weighting and averaging operation is performed in accordance with the reception level of the reflection wave. Thus, it is possible to divide balancing processing in the line so that the azimuth angle of the reflector is detected.
As the resolutions of distance and azimuth angle are improved, a detection area or a field of view of the radar apparatus in the front of a vehicle can be divided into meshes in distance and in azimuth angle (the width) to form sub-areas of a two-dimensional structure. Thus, the reflector such as a preceding vehicle is detected over the sub-areas.
According to Japanese Laid Open Patent Application (JP-A-Heisei 5-180934), a group of continuous sub-areas in which the reflection wave is detected, is detected as a single reflector wave so that the movement of the reflector in the detected sub-areas is managed so as to detect a relative velocity between the subject vehicle on which the radar apparatus is installed and the reflector such as another vehicle or an obstacle. In the method of estimating the relative velocity between the subject vehicle and the other vehicle, the reflector such as the preceding vehicle as a detection object is managed only in the field of view of the radar apparatus installed on the subject vehicle. Therefore, a perfect data on the shape of the reflector can be obtained in a tolerance limit of the spatial resolution, when the reflector is detected in the center portion in the field of view of the radar apparatus. However, when the reflector is detected in a peripheral portion in the field of view of the radar apparatus, only an imperfect data can be obtained on the shape of the reflector due to lack of data for a portion out of this field of view. Therefore, when the reflector, which has been detected in the center portion in the field of view of the radar apparatus, is about to leave out of this field of view, the perfect data of the center position and width is once acquired and then is lost.